Kyiv launched its pushback in June after stockpiling Western-supplied weapons, building up assault battalions and attacking Russian positions.

But officials in Kyiv have acknowledged that progress has been slow and Moscow says Ukrainian forces are running out of resources.

"Robotyne has been liberated. Our forces are advancing southeast of Robotyne and south of Mala Tokmachka," Deputy Defence Minister Ganna Malyar said on television.

Both settlements are in the southern region of Zaporizhzhia, one of four that the Kremlin claimed to have annexed last year despite not having military control over any of them.

Ukraine's limited advances on the southern front have spurred a political debate in Western capitals over political and military support for Kyiv.

Compared to previous offensives in the Kherson and Kharkiv regions, Ukrainian forces are crashing into Russian defensive lines of trenches and minefields that are kilometres deep.

But experts say that the capture of Robotyne is evidence Ukrainian forces can puncture Russian lines as they move towards the Black Sea.

- 'Intense' fighting -

The US-based Institute for the Study of War said Russia had "committed a considerable amount of materiel, effort, and manpower to hold the series of defensive positions that Ukrainian forces are currently penetrating".

Apart from clawing back several hamlets in the south, Ukrainian forces have pressured the flanks of Bakhmut, a war-scarred eastern town captured by Russia in May.

Malyar said Monday that Ukrainian troops were advancing south of Bakhmut and that they had recaptured one square kilometre (around one-third of a square mile) there over the last week of fighting.

She also acknowledged a Russian push to take back territory in the northeast of Ukraine, describing fighting in the Kharkiv region as "very intense" over the past week.

Ukrainian officials have estimated that Russia has committed around 100,000 additional troops to frontlines in the northeast to pressure defensive lines.

British intelligence services have said Russia could try to divert Ukrainian military resources by solidifying their positions in Kharkiv.

While movement on the frontline has been stilted, both sides have pursued aerial assault campaigns.

- Drone attacks -

A Russian missile strike overnight on an industrial facility in the central region of Poltava left three dead and five wounded, the police said.

Images released by Kyiv showed law enforcement officials standing next to a large blaze engulfing part of a factory with plumes of black smoke billowing into the sky.

Presidential chief of staff Andriy Yermak said on social media that Russian forces had hit an oil facility in the town of Gogoleve and that two employees were killed.

The air force said Russia had launched six cruise missiles and that air defence systems had downed four of them.

Russia's defence ministry said those strikes had targeted a weapons depot and that the assigned targets had been hit.

Russian authorities earlier said its systems had destroyed a drone approaching Moscow and two in a region bordering Ukraine.

Russian-installed authorities in Crimea, which Moscow annexed in 2014, later said two Ukrainian drones had been shot down over the peninsula.

The head of Russia's election commission, Ella Pamfilova, announced Monday that early voting in local elections in the four newly annexed territories as well as Crimea would begin this week.

Previous ballots in occupied territories have been dismissed by Kyiv and its Western allies. Russia is holding local elections on September 10 for various regional officials.